Both natural and synthetic elastomers usually require the use of processing aids to assist mechanical breakdown and compounding. Materials such as mixtures of oil soluble sulfonic acids of high molecular weight with a high boiling alcohol, paraffin oils, blends of sulfonated petroleum products and selected mineral oils are conventionally used as processing aids. Additional examples include petroleum, paraffinic and vegetable oils, coal tar, petroleum residues or pitches and naturally occurring or synthetic resins.
One advantage in using processing aids is they assist the incorporation of fillers and other ingredients with low power consumption since they reduce internal friction in calendering and extrusion. By reducing the amount of friction during compounding, the temperature of the rubber will remain lower and thus minimize the possibility of scorch.
Various types of rosin acids have been used as extenders for high molecular weight SBR. See Properties of GR-S Extended With Rosin Type Acids, L. H. Howland, J. A. Reynolds, and R. L. Provost, Industrial and Engineering Chemistry, Vol. 45, No. 5, May 1953. Whereas reasonably good cured physical properties can be obtained with the rosin type acids, there are problems associated with their use which include cure retardation, high tack and poor low temperature performance, which limit their use as an extender in rubber formulations.
U.S. Pat. No. 4,491,655 discloses the use of methyl esters of rosin acid as total or partial replacement for oil in a rubber formulation. Compared with the use of aromatic extending oils in rubbers, methyl esters of rosin acids provide comparable processing and low temperature performance and superior abrasive resistance and flex resistance. Unfortunately, use of methyl esters of rosin acid does not benefit the vulcanizate properties related to rebound and tear. These properties are typically improved by the supplemental addition of a multitude of additives. The cost of compounding all these additives as well as the potential and detrimental interaction of these additives is preferably avoided. Therefore, there is a need for a single additive which can improve a number of properties while decreasing the cost and detrimental interaction by the addition of a multitude of compounds.